Just wondering...if any of the "dust" that accumulates on the D600 sensor is actually debris from moving parts in the camera (as many have suggested), than isn't it metal and won't wiping the sensor clean lead to small scratches in its surface? Or is it true that the surface is a replaceable low pass filter (to inhibit moire)?

Silica dust (a.k.a. fine sand) might cause sleeks. That is why it is recommended that you only take one swipe per side of a swab pad before changing the pad.

There is no problem replacing the dust shaker which is the surface that gets cleaned. Depending on the design, some Nikons have two IR-blocking filters. One is the dust shaker and the other is the actual low-pass filter which sits between the dust shaker and sensor.